Glass-sealed resistor



March 22, 1960 J. M. HlNKLE GLASS-SEALED RESISTOR Filed June 15, 1954 scribed in detail in disclosure nitccl States Paten 2,930,018 GLASS-SEALED RESISTOR John M. Hinkle, Morris Plains, NJ. Application June 15, 1954, Serial No. 436,809 .3 Claims. (Cl. 338 237) possess the required electrical properties, such as plati-' num-gold alloy, but carbon is so commonly used for the deposited resistive conductor film that a carbon film resistors has been chosen as the illustrative example to be deof the present invention, which has particular reference to,the means employed to hermetically seal the delicate resistive conductor film against exposure to physical and chemical damage.

Prior to the present invention, it has been common practice to seal the resistor in a glass envelope that completely encloses it and is spaced radially from the resistive conductor film. If the film were exposed to air, decomposition would occur, so the space between the film and envelope customarily is filled with a pure inert gas, such as helium.

The prior art sealing structure 7 just mentioned possesses numerous disadvantages.

The over-all size is objectionably large for uses in which miniaturizationis almost mandatory. The gas atmosphere surrounding the resis tiveconduct'or film affords relatively poor heat dissipation, resulting in a comparatively long thermal time constant, and produces changes in the state of sorption of the film surface and damage from gas ionization, which latter also has the effect of limiting the high voltage capabilities of the resistor. Still other disadvantages of the earlier glass sealing structure are lack of ruggedness, the dilficulties of installing leading-in wires,'-and the necessity for making the sealed resistors one at a time. Consequently, due to the amount of hand labor required,'production costs have been very high.

It, therefore, is my'primary object to provide a glass seal for a resistor in the deposited carbon film category in an improved manner which renders the product free from the above-enumerated objectionable features of the earlier gas-containing glass envelope.

To be more explicit, in one embodiment of theinvention, my improved sealing medium is produced by shrinking an enclosing glass tube in heat-softened condition into such close contact with the coreisuppor'tedresistive conductor film that all air or other ga es willbe excluded from between the glass andcarbon layers. In another embodiment, the glass tube is shrunk into contactwith the, end portions only. Both the tube shrinking and the gas exclusion are facilitated by connectingthe tube with a vacuum line while heat is being applied. l V w I I am aware that. others have applied a contactcoating of varnish to the carbon film of a resistor to produce a gasexcluding sealing medium, but use ofany such organic material is objectionablebecause, of the deleteriousteffect onthe carbon film of. gases evolved by decomposition of the organic material under the influence of the high tem- 2,930,018 Patented jMar. 22, 1960 peratures to which a resistor often is subjected in opera- The sealing coat thus produced was observed to be bubbled and probably was subject to pinhole difiiculties. Itf YVQS type, in the construction of which the resistion in operational use.

7 then that I conceived the idea of inserting the resistorin'a glass tube and shrinking the latter into tight gas-exclud ng contact with the-peripheral 'surfaces of the resistor."

Another object of the inventioniis toprovide a glass sealing coat for the resistor that leaves the ends of the latter exposed for direct connection of lead wires without having theproblem of leading them in through the sealing medium in a hermetically sealed manner. This provision 20' introduced the'problem of insuring against opening of the joint between the edges of the sealing coat and the resistor under the influence of thermal expansion and contrac- It is therefore a further object of the invention to prevent failure'of the seal under any such conditions, which has been accomplished by *u'se'bf novel contact caps at the opposite ends of the resistor which co'operate with the glass sealing coat to providea highly eliectiveendseal. Other objects, advantages and features of the invention will become apparent as the following specific description is read in connection with, the accompanying drawings} in which:

1 Fig. lis a side elevation, partly insection and'onla greatly enlarged scale, of a typical carbon resiston-showing the same in its primary elemental -form prior tof the condition. g

' Fig. 3 is a view similar to Figs. 1 and 2, showing athin completion of the film scoring operation but in coating of solid electrically conductivemetal covering the end portions of the carbon film in tion of contact caps; Fig. 4 is a contact caps in place; and'Fig.

preparation for applicasimilar view showing the 5 is a similar view of the completed resistor, showingthe glass sealing wall in shrunkon coextensive peripheral contact with the carbon film and contact caps andalso showing the lead wires 7 inwelded connection with the end walls of the contact caps.

. Referring now in detail to the drawings, in whichlike reference characters designate corresponding parts inthe several views, Figs. 1 and 2, respectively represent atypical, carbon resistor 10 in its successive elemental forms. The process of manufacturing such a resistor starts with a blank to serve as a base body or core upon which a coating or film of carbon maybe supported to function as the resistive conductor thereof.

Fig. 1 shows the basebody or corellafter deposition of 'its'carbon coating or'filrn 12, which covers all exterior surfaces, both'peripheral and at the ends. Carbon film 12 is almost microscopically thin and may beappliedby a process that has beenused for many years in thiis and other countries.

This process consists in exposing the core blank toan atmosphere of a dilute hydrocarbon vapor, such as methane or benzene, at a suitably. elevated temperature. I prefer to use high purity nitrogen to dilute methaneand coat at about 1100 C. Sever-a1 blanks may be coated as abatch in ages-tight bottle which is rotated to expose the blanks uniformly as the coating gas' is passed through the bottle. However, they may be coated singly in a suitable The structure and composition of the core of each resistor are important. Preferably, the core 11 is 1n the form of 'a cylindrical rod of good mechanical strength and made from a material which possesses the electrical propertyof high insulation resistivity and which is sulficiently refractory to withstand high temperatures. The

selected material also should be of such composition that the finished blanks will be non-porous and have a very high degree of surface perfection in order to permit the deposition thereon of a uniform layer of carbon which will insure the stability of the resistor. The surface,

therefore, must be free from pinholes, cracks, blebs,

iron spots, and heterogeneities of any sort, as well as being free from scratches, die marks, and the like. Moreover, the surface should be smooth, but of microscopic "roughness which mechanically keys deposited carbon to the surface and thus provides excellent adherence.

A ceramic material of the porcelain type disclosed in US. Patent No. 2,386,633, dated October 9, 1945, meets 1 all of the above-enumerated requirements for the carbon resistors now in general use, and, at least one of the several exemplary combinations of chemical ingredients described in that patent possesses thermal expansion properties well suited to the matching of expansion coefficients which is involvedvin the choice of materials for use in composition of the core, contact caps and sealing medium .of my improved resistor, as will be explained more fully later herein. I

While the resistor in its primary elemental form represented in Fig. 1 is quite capable of operational use, the effective length of the resistive conductor constituted by carbon film 12 will be relatively short and the ohmic value of the resistance interposed thereby in an electric circuit. will be proportionately low. Therefore, it is customary to increase the eflective length of the resistive conductor by cutting a helical groove 13 through carbon film 12 into core 11 to transform the interior peripheral portion .of the film into a helical strip in the manner shown in Fig. 2. The axial extent of groove 13 should be such as to leave undisturbed areas of film 12 adjacent to the ends of resistor for application of terminal contact means, which latter, in accordance with prior practice, sometimes have been in the form of encircling bands of conducting metal (not shown).

"Up to this. point in the disclosure, all physical and chemical details of the two embodiments of the elemental resistor illustrated in Figs. 1 and 2 are old and well known in the art. However, the details of structure and chemical composition disclosed in the subsequent figures of drawing are either novel in themselves or are inventively combined with novel details.

-between the carbon surface and the lead wires to insure low contact resistance and consequent electrical stability. The contact caps also hermetically seal the terminal portions of resistive conductor 12.

Preparatory to installation of the contact caps, the end faces and adjacent ungrooved peripheral portions of the carbon film 12 are given a microscopically thin coating of a metal possessing high electrical conductivity. Although silver or some other metal which satisfies this requirement may be used within the scope of the invention, it is preferred to use copper. Organic paint pigmented with metal flakes might be used to form the metallic coating, but the presence of any organic substance inside the finally applied glass sealing coat or wall is objectionable due to the release of deleterious gases upon inevitable decomposition of the organic vehicle in which the metal flakes are suspended by the heat to which oven. Thedesired carbon film generally is from 10- cm. to 10" cm. in thickness.

core 11 is composed and the particular glass to be used in formation of the sealing medium. For use with a the resistor will be subjected during formation of the glass seal and in subsequent operational use. Instead, it is preferred to apply a deposited coating of pure solid copper by successively exposing the respective end portions of the resistor in its Fig. 2 form to evaporation of copper in an evacuated bell jar or equivalent chamber. Fig. 3 shows the preparatory copper coatings 1414 at the opposite ends of carbon film 12.

Fig. 4 shows the resistor after cylindrical contact caps 1515 have been force-fitted onto the copper coated areas 14-14. Each contact cap includes integral side and end walls which must be imperforate so that no air or other gas can possibly penetrate to the carbon film 12 through the end wall thereof. Gas penetration in peripheral areas is prevented by the glass sealing medium to be described in detail presently.

In accordance with the present invention, contact caps 15 are composed of a metal possessing high electrical conductivity which also has substantially the same coefiicient of linear expansion as the material of which core composed of porcelain type ceramic, such as is disclosed in US. Patent No.2,386,633, I prefer to use a metallic alloy containing approximately 29 percent nickel, 17 percent cobalt, .3 percent manganese, and 53.7 percent iron in construction of contact caps 15. To match the thermal expansion coefficients of the selected core and contact cap compositions, the sealing medium may be composed of a hard boro-silicate glass.

Although the matching of thermal expansion coefficients just described should insure a gas-tight joint between the glass sealing wall and the peripheral surfaces of plain-surfaced contact caps, I have devised a precautionary measure which absolutely precludes any gas penetration in this area and which consists in oxidizing the peripheral surfaces of contact caps 15 for cooperation with the fused glass during installation of the sealing coat or wall. To avoid objectionable oxidation of the inner surfaces of the contact caps, where perfect electrical conduction is desired, the problem was solved by copper-plating one side only of the sheet metal blank from which the contact caps were stamped. Then, by stamping out the caps in such a manner that the copper plated surfaces were on the interior, it became practicable to oxidize the outer peripheral surfaceswithout affecting the plain inner surfaces by use of a wet hydrogen atmospherc.

Fig; 5 illustrates the completed glass-sealed resistor 10 of my invention in its preferred form wherein the shrunkon glass wall adheres closely to the carbon film 12 and the exposed areas of core 11 within grooves 13 as well as tothe peripheral surfaces of contact caps 15. ,In an alternative embodiment of the invention, glass sealing tube 16 is shrunk into close contact with the oxidized caps 15 only while maintaining radial separation between the said tube and core 11 with its carbon film 12 (not shown).

While I do not wish to be limited to the use of any particular combination of specific chemical compositions for the three principal mechanical constituents of my improved glass-sealed resistor, there are two other corecap-glass systems with matching thermal expansion coefiicients which may. be suggested'as possible alternatives. One is a hard glass system, like the one previously disclosed herein, and the other is a soft glass system.

The alternative hard glass system comprises a core of dense alumina ceramic, contact caps of molybdenum, and bore-silicate glass; whereas the soft glass system comprises a core of steatite, contact caps of an alloy containing 42 percent nickel, 52 percent iron and 6 percent chromium, and potash-soda-lead glass.

Incidentally, all elements of the three core-cap-glass systems disclosed herfiin are commercially available.

It will be understood that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.

Having thus described the invention, I claim:

1. A hermetically sealed resistor comprising: an elongated cylindrical core composed of a refractory material of high electrical insulation resistivity; a thin film-like resistive conductor extending substantially coextensively lengthwise on the peripheral surface of said core; impervious cylindrical contact caps of electrically conductive metal fitted on the respective ends of the core in snug air-excluding telescopically enclosing electrical contact with the terminal portions of the resistive conductor throughout substantially the entire length of each contact cap, each of said contact caps including a flangeless cylindrical tubular side wall and an integral closing wall at one end only, said closing end wall being outwardly presented to provide an impervious end seal for the core and resistive conductor; and a tubular shrunk-on sealing wall of vitreous electrically non-conductive maten'al snugly and air-excludingly enclosing the core, the resistive conductor and the side Wall only of the contact caps substantially throughout the entire length of said caps.

2. A hermetically sealed resistor as defined in claim 1, wherein the core is composed of dense alumina ceramic; the contact caps are composed of molybdenum; and the sealing wall is composed of hard boro-silicate glass, whereby the core, contact caps and sealing wall possess substantially the same coefiicients of linear expansion to prevent separation under the influence of thermal changes.

3. A hermetically sealed resistor as defined in claim 1, wherein the core is composed of steatite; the contact caps are composed of an alloy containing approximately 42 percent nickel, 52 percent iron and 6 percent chromium; and the sealing wall is composed of soft potashsoda-lead glass, whereby the core, contact caps and sealing wall possess substantially the same coeflicients of linear expansion to prevent separation under the influence of thermal changes.

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